Insulating materials having temperature-dependent electrical resistance or capacitance characteristics have long been extensively used in overheat sensing and control applications. Thus, by virtue of the invention of Spooner and Greenhalgh disclosed and claimed in U.S. Pat. No. 2,581,212 overheat protection for electric blankets and similar articles is provided for the use of such materials to afford the essential safety factor. In accordance with the teachings of that patent, the insulating material is operatively associated with switch means and is coextensive with the heating element so that when the temperature anywhere in the blanket exceeds a predetermined maximum, the blanket heating power supply is interrupted. Because this insulating material is not altered physically or otherwise irreversibly changed in so functioning, it is useful repeatedly for this purpose as it acts as a sort of electrical switch constantly monitoring the blanket operating temperature limit.
A variety of insulating materials are identified in the prior art as being suitable for such use. Those include in addition to the preferred Nylon polyamide resin of the aforesaid patent, polymeric organic materials such as polyvinyl chloride and cellulose esters containing additives imparting the desired electrical characteristics. In U.S. Pat. No. 2,745,944 to Price, still another kind of material for this same purpose, sulfur-cured butadiene-acrylonitrile elastomer is disclosed. That material and all the others of the prior art, however, are in one respect or another, less than what has been desired and general recognition of that fact has failed heretofore to result in a thermal-sensing insulating material approaching the ideal which would combine the best properties and characteristics of each of those, but would be free, at least to a large degree, from their major drawbacks which are relatively low levels and ratios of changes in impedance with temperature and, in the case of DC volume resistivity, high levels of volume resistivity and low ratios of changes in volume resistivity to temperature. In addition as in the case of Nylon resin, the effect of humidity shifts the levels of impedance resistivity to the extent that control circuits become a problem.
The practical significance of such shortcomings of prior art thermal-sensing materials is apparent from the commercial electric blanket experience. Thus when exposed to moisture, the Nylon insulation temporarily loses its desirable electric properties to a large extent in only an hour or two even though the insulation in an electric blanket is covered by a layer of polyethylene and an overlayer of polyvinyl chloride as described in the Spooner and Greenhalgh patent reference above.
The catastrophic effect of the presence of free sulfur in thermal-sensing insulation on wires of the type used in electric blanket structures has been demonstrated in tests under normal operating conditions running only six hours to wire failure.
U.S. Ser. No. 709,245 now U.S. Pat. No. 4,616,124 (Greenhalgh) filed Mar. 7, 1985 incorporated herein by reference discloses a new class of thermal-sensitive insulating compositions which applicant has discovered are particularly suited for use in smoke and/or heat detectors. Broadly speaking, the compositions are comprised of an admixture of a polymeric material, a filler, a plasticizer, etc. in proportion to optimize the desired electrical, physical and processing characteristics to achieve the product.
The polymeric material selected should contain substantially no free sulfur. In the case of the acrylonitrile butadiene formulations, the acrylonitrile must be present in an amount of at least 1%; and for the carboxylated material, the acid monomer units should be present in an amount of at least 0.5%.
In addition to being curable by either sulfur-bearing combinations or peroxide, the carboxylated polymer can be cured by zinc oxide. Further, both polymers (carboxylated and non-carboxylated acrylonitrile butadiene) can be used as plasticizers for such a resin as polyvinyl chloride.
In practice, as indicated above, acrylonitrile butadiene rubber of the relatively high acrylonitrile type which has S.I.C. ratios (90.degree. C. to room temperature) of the order of 10 or more are employed. Those materials preferably contain about 20% to 45% acrylonitrile by weight. Also, preferably acrylonitrile butadiene rubbers contain carboxyl groups which further enhance the desired electrical properties of interest, these being introduced by copolymerization with acrylonitrile and butadiene commonly derived from acrylic acid, methylacrylic acid, maleaic acid or the like. Preferably, the amount of carboxyl groups is more than the minimum of 0.5% by weight. Suitable polymers available on the market are set out in Table I.
TABLE I ______________________________________ % Acrylo- % Butadiene Nitrile % Carboxyl ______________________________________ Goodyear NX775 68 26 6 Goodrich 1072 67 27 6 Polysar 110C 64 32 2 Polysar 231C 59 34 7 ______________________________________
As with acrylonitrile butadiene elastomers, the curing system involves sulfur in the free state, sulfur bearing in which sulfur is available in combined form, and peroxide. In addition, carboxylated acrylonitrile butadiene combinations may be cured with a metallic oxide such as zinc oxide which is the preferred curing system. The amount of zinc oxide for this purpose may be from 1 to 10 pts to 100 pts. of elastomer.
In addition to the superior desired electrical properties carboxylated elastomers in the cured state have the additional attributes of increased hardness, tensile strength, ozone resistance, and abrasion resistance.
When it is not possible to cure the polymeric material on a conductor, blends of either the acrylonitrile butadiene or the carboxylated acrylonitrile butadiene may be used in combination with a suitable resin such as polyvinyl chloride. In either cure, the elastomer acts as a migratory plasticizer and the mixture is considered to be pure thermoplastic. The preferred ratios of resin to elastomer are in the range of 1 to 4 to 1 to 1, respectively.
The combinations referred to above when containing a carboxylated elastomer have the additional advantage of retaining the inherent properties of the carboxylated elastomer even in the uncured state.
Additionally, clay, and particularly a kaolin such as Catalpo clay (Freeport Kaolin Company trademark) enhances the S.I.C. ratio and the volume resistance.
A further application is disclosed in U.S. Ser. No. 847,481 (Greenhalgh) filed Apr. 3, 1986 incorporated herein by reference wherein temperature fluctuations in power cables are monitored using the same polymeric material as disclosed in U.S. Ser. No. 709,245 or in Ser. No. 858,351 filed as of even date herewith, both of which are incorporated herein by reference.
The construction and operation of smoke detectors and heat (fire) detectors and devices containing both features are well known.
For example, single and dual ionization chambers are disclosed in "Ionisation Type Smoke Detector Technology" by Michael Byrne May 13, 1981, incorporated herein by reference. In a single chamber design, a radioactive source emits alpha particles to disloge electrons from air molecules to thereby produce positive ions. The electrons almost immediately attach themselves to other molecules thus producing negative ions.
When a voltage is applied between the electrodes these ions will drift under the influence of the electric field and so constitute a current. If smoke particles enter the region between the plates some of the ions will become attached to them by diffusion. This causes a reduction in the current flowing because the smoke particles are far too massive for the electric field between the plates to deflect them appreciably. This decrease in current is detected and used to trigger the alarm.
The dual ionization chamber design contains a smoke sensing chamber which has access to the ambient air and another, reference chamber, which is almost completely sealed. The two chambers are connected in series. The relative geometries of the chambers are arranged such that the reference chamber, because the voltage across it is high enough, operates in the saturation region of its characteristic. The smoke chamber is operated in its linear region as this is where it was most sensitive to smoke. When smoke enters the chamber the ion concentration (contributing to the ionization current) is reduced. However, because the reference chamber, from an electrical point of view is essentially a constant current source, the voltage across the smoke chamber increases, thereby providing the means for the generation of an electrical signal in response to the presence of smoke and/or heat.
Improvements in ionization smoke detectors have been made in recent years as disclosed for example, in U.S. Pat. Nos. 4,185,196, 4,185,197 and 4,220,262 all incorporated herein by reference.
Other types of smoke and/or heat detectors are familiar to those skilled in this art.
Despite all of the recent attention to smoke and/or heat detectors there is still a need for improving the electrical insulating capabilities of conductors employed in such devices so that the devices have improved levels and ratios of changes in impedance with temperature and, in the case of DC-volume resistivity, high levels of volume resistivity and low ratios of changes in volume resistivity to temperature. In addition, improvements in resistance to humidity changes to thereby reduce shifts in the levels of impedance resistivity are also desired.
It is therefore an object of the invention to provide an improved smoke and/or heat detector which is sensitive to low levels of heat and smoke and more resistant to changes in humidity levels.
It is a further object of the invention to provide a smoke and/or heat detector with improved temperature/impedance ratios.